Optical fiber networks lie at the core of modern telecommunications. As the cost of optical fibers and associated components decreases, network architectures increasingly use optical fibers from the edge of a core network to a location at or very close to the end users. Such implementations are referred to as “fiber to the X” (FTTX), where X can stand for the home (FTTH), office (FTTO), building (FTTB), curb (FTTC), premises (FTTP), etc. For cost considerations, FTTX solutions are generally deployed with passive optical network (PON) architectures in which data, voice, video and other services are conveyed to the end users through passive splitting, rather than active switching, devices.
A PON generally consists of one or more optical line terminals (OLTs), typically located in a service provider's central office, a number of optical network terminals (ONTs) or units (ONUs), typically located near the end users, and an optical distribution network (ODN) including optical fibers to connect the OLTs to the ONTs and supplemented with power and wavelength splitters, filters and other passive optical devices. Many PON protocols have been developed by standard bodies such as the International Telecommunication Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE). Non-limiting examples of PON protocols include: asynchronous transfer mode PON (APON); broadband PON (BPON); gigabit-capable PON (GPON); Ethernet PON (EPON); 10-gigabit-capable PON (10G-PON or XG-PON); and next-generation PON 2 (NG-PON2). It is to be noted that for simplicity, the term “ABG-PON” will be used herein to encompass APON, BPON, GPON and other older-generation “legacy” PONs that use the 1490 nanometer (nm) wavelength for downstream traffic and the 1310 nm wavelength for upstream traffic.
PONs involve bidirectional single-fiber communication between pairs of network elements, in which one network element in each pair is configured to interrupt signal transmission if the optical link between the two network elements is broken. Because of this, optical power measurement of PON signals is generally performed with special-purpose or dedicated power meter instruments. These instruments are configured to ensure that transmission of one of the communication signals is maintained while attempting to measure the optical power of the other, counterpropagating signal.
FIG. 1 illustrates an example of a conventional PON power meter (PPM) design suitable for legacy PONs. Other implementations of such a PPM design are described, for instance, in U.S. Pat. Nos. 7,187,861; 7,995,915; 8,861,953; and 9,287,974, the disclosures of which are incorporated herein by reference in their entirety. However, while the PPM design shown in FIG. 1 can be advantageous in many applications, it is generally unable to distinguish among multiple downstream OLT signals propagating in different data-carrying wavelength-division-multiplexing (WDM) channels (aside from a possible signal centered near 1550 nm, for example a CATV signal, which can be selected with a bandpass optical filter). This PPM design is also generally unable to identify the particular downstream WDM or dense WDM (DWDM) wavelength associated with the ONT for which power measurements are performed.
In NG-PON2, the optical transmission path between an OLT and an ONT may carry multiple downstream optical signals in respective WDM channels. However, the number of these downstream signals propagating in a given optical transmission is, in general, not known precisely. Also, despite the presence of multiple downstream WDM channels, generally only a single one of these channels is actually read or listened to by the ONT, the wavelength of which being generally unknown or not readily accessible to the PPM and/or the operator tasked to troubleshoot or assess conformity of the communication link between the OLT and the ONT. In such a case, conventional legacy PPMs are limited to measuring only the total optical power carried by all of the downstream optical signals forming the downstream light, which is generally insufficient to confirm whether the WDM channel actually read by the ONT is present, let alone to indicate its optical power.
Furthermore, PPM designs such as shown in FIG. 1 are generally not well adapted for optical power measurement in next-generation, multiple-wavelength PON systems, such as NG-PON2, involving both time and wavelength division multiplexing (TWDM) in both the downstream and upstream directions. The NG-PON2 architecture is specified in the ITU-T G.989 family of recommendations, including ITU-T G.989.1 and G.989.2. For example, the ITU-T G.989.2 recommendation specifies different operation modes for NG-PON2, including a TWDM PON operation mode, in which each ONT may communicate with multiple OLTs, and a point-to-point (PtP) WDM PON operation mode. For some applications, a PPM suitable for NG-PON2 networks may need to accommodate both of these two operation modes.
NG-PON2 is also backward-compatible with legacy PON architectures such as GPON and XG-PON, RF video overlay, and optical time-domain reflectometer (OTDR) measurements (see, e.g., the ITU-T G989.2 recommendation for NG-PON2, as well as the ITU-T G.984 and ITU-T G.987 families of recommendations for GPON and XG-PON, respectively). In particular, different legacy PON architectures and different NG-PON2 architectures can coexist on a given PON. In this context, it would be inconvenient, time-consuming and/or error-prone to require the operator to reconfigure a PPM before each measurement in accordance with the particular PON architecture at the OLT currently being tested.
Accordingly, various challenges remain in the development of PPMs that can allow optical power measurement of communication signals in multiple-wavelength PON systems.